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Development of a robotic cell for the printing of
electronic circuits on free form surfaces and industrial
applications
Gioia Furia
To cite this version:
Gioia Furia. Development of a robotic cell for the printing of electronic circuits on free form surfaces and industrial applications. Mechanics of materials [physics.class-ph]. Université Grenoble Alpes [2020-..], 2021. English. �NNT : 2021GRALI015�. �tel-03228497�
FURIA Gioia 3
TABLE OF CONTENT
GENERAL INTRODUCTION
1. CONTEXT OF THE PROJECT ... 11
2. OBJECTIVES OF THE THESIS ... 13
3. MAIN OBSTACLES AND THESIS STRUCTURE ... 14
4. BIBLIOGRAPHY ... 16
5. TABLE OF FIGURES ... 16
CHAPTER 1: BIBLIOGRAPHY INTRODUCTION ... 23
1 HIGH PRECISION FABRICATION PROCESS ... 24
2 Robotic arm: architecture and cinematic ... 24
2.1 Manufacturing process ... 25
2.2 Main issues causing inaccuracies ... 26
2.3 Inaccuracies related to the object ... 27
2.3.1 Inaccuracies linked to the process ... 28
2.3.2 Inaccuracies related to the static accuracy of the robot ... 28
2.3.3 Mesure in-situ ... 30
2.4 Position of the measuring phase in the manufacturing process ... 30
2.4.1 Integration of the measuring equipment in the working area ... 31
2.4.2 FUNCTIONAL MATERIALS PRINTING ... 33
3 Direct and contactless printing process ... 33
3.1 Aerosol ... 34 3.1.1 Inkjet ... 35 3.1.2 Jetting ... 36 3.1.3 Extrusion ... 36 3.1.4 Comparison ... 37 3.1.5 Physical and chemical properties ... 38
3.2 Physico-chemistry of the ink ... 38
3.2.1 Rheological behaviour of the ink ... 40
3.2.2 Conductive property of printed tracks ... 42
3.2.3 Metallic inks... 43
3.2.4 Carbon based ink ... 46
3.2.5 Conductive polymers ... 47 3.2.6
FURIA Gioia 4 Substrates used in electronic printing ... 48 3.3
Substrates characteristics ... 48 3.3.1
Substrates studied in literature ... 50 3.3.2
Molded cellulose ... 51 3.3.3
Micro Fibrillated cellulose (MFC) ... 52 3.3.4 Annealing types ... 52 3.4 Thermal annealing ... 53 3.4.1 Chemical annealing ... 53 3.4.2 Electrical annealing ... 54 3.4.3 Plasma annealing ... 55 3.4.4 Microwave annealing ... 56 3.4.5 Photonic annealing ... 56 3.4.6
CONNECTED OR FUNCTIONAL OBJECT ... 60 4
Molded Interconnect Devices (MID) ... 60 4.1
Fabrication process ... 60 4.1.1
Research project examples ... 62 4.1.2
Industrial application examples ... 62 4.1.3
Additive manufacturing of 2D multilayer functional devices ... 63 4.2
Paper microfluidic ... 63 4.2.1
Papertouch ... 64 4.2.2
Additive manufacturing of 3D functional objects ... 65 4.3
Multimaterials objects additive manufacturing process ... 65 4.3.1
Industrial application examples ... 67 4.3.2
Robotic for 3D printing ... 69 4.4
Research project examples ... 69 4.4.1
Industrial application examples ... 71 4.4.2 CONCLUSION ... 74 5 BIBLIOGRAPHY ... 75 6 TABLE OF FIGURES ... 83 7 TABLE OF TABLES ... 84 8
CHAPTER 2: ROBOTIC CELL AND OFF-LINE PROGRAMMING SOFTWARE DEVELOPMENT
INTRODUCTION ... 90 1
3D SIMULATION AND POST-PROCESSOR SOFTWARE ... 91 2
FURIA Gioia 5 VAL 3 language ... 92 2.1
Structure of VAL3 language: application and program. ... 92 2.1.1
Control of movement ... 92 2.1.2
Presentation of simulation and off-line programming tools ... 95 2.2
Stäubli Robotics Suite (SRS) ... 96 2.2.1
Commercial industrial tools ... 96 2.2.2
Rhinoceros 3D plugin ... 97 2.2.3
Comparative table ... 98 2.2.4
Choice of a simulation and off-line programming tool ... 99 2.3
Generation accuracy of the object in the working environment ...101 3
Mesh generation methods: bibliography focus ...101 3.1
Scanning tools ...102 3.1.1
From points cloud to mesh generation ...104 3.1.2
Mesh quality evaluation ...109 3.2
Process implementation and characterisation ...112 3.3
Scan step implementation...112 3.3.1
Reverse engineering step implementation ...120 3.3.2 Process validation ...127 3.4 Process description ...127 3.4.1 Examples ...128 3.4.2 Criteria validation ...134 3.5
3D ELECTRONIC CIRCUITS PRINTING ...137 4
Electronic circuit printing on 3D objects: bibliography focus ...137 4.1
The CAD model of the part on which the material will be deposited ...137 4.1.1
The chosen tool ...137 4.1.2
The path pattern ...138 4.1.3
The process requirements and parameters ...138 4.1.4
Required printing quality ...139 4.2
Projection process ...139 4.3
Printing process ...142 4.4
PRINTING ROBOTIC CELL ...143 5
Cell requirement ...143 5.1
Schematic diagram and description ...143 5.2
3D Simulation environment and interface description ...145 5.3
Cell criteria validation ...152 5.4
CONCLUSION ...153 6
FURIA Gioia 6 BIBLIOGRAPHY ...154 7 TABLE OF FIGURES ...157 8 TABLE OF TABLES ...158 9 CHAPTER 3: APPLICATIONS INTRODUCTION ...164 1 PRINTING ON 3D OBJECTS ...165 2
Printed lines characterisation ...165 2.1
Printing tool implementation ...165 2.1.1
Robot speed analysis...166 2.1.2
2D Printing tests ...169 2.1.3
Predictive model creation...170 2.2
Model theory ...170 2.2.1
Analysis and results...171 2.2.2
Printed line conductivity ...174 2.2.3
Predictive model and quality validation ...176 2.3 Process description ...176 2.3.1 Example ...177 2.3.2 Criteria validation ...180 2.4
Precise control of the number of drops deposited ...181 2.5
Conclusion...183 2.6
2D MULTI-MATERIAL APPLICATIONS: USE FOR THE MANUFACTURING OF 3
ENCAPSULATED MICROFLUIDIC DEVICES ...185 Spontaneous capillary flow ...185 3.1
Capillary force ...185 3.1.1
Dynamic of spontaneous capillary flow...186 3.1.2
Manufacturing of paper microfluidic medical diagnostic devices ...187 3.2
Prerequisite for a medical diagnostic tool ...187 3.2.1
State of the art of the manufacturing of paper microfluidic devices ...189 3.2.2
Developed manufacturing process ...190 3.2.3
Development of the required functionalities ...193 3.3
Paper spray coating ...193 3.3.1
Capillary system...197 3.3.2
Heating system ...204 3.3.3
Towards a point of care diagnostic medical devices ...211 3.4
FURIA Gioia 7 CONCLUSION ...212 4 BIBLIOGRAPHY ...214 5 TABLE OF FIGURES ...217 6 TABLE OF TABLES ...218 7 GENERAL CONCLUSION CONCLUSION ...222 1 PERSPECTIVES ...224 2 RÉSUME ÉTENDU INTRODUCTION ...229 1
DÉVELOPEMENT D’UNE CELLULE ROBOTISÉE POUR L’IMPRESSION DE CIRCUITS 2
ÉLECTRONIQUES ...232 Réalisation de la cellule robotisée ...232 2.1
Description de la cellule ...232 2.1.1
Développement du post-processeur ...234 2.1.2
Développement du processus d’impression ...235 2.2
APPLICATIONS ...238 3
Impression de pistes conductrices sur des objets 3D ...238 3.1
2.1.1 Tests d’impression : matériel et méthode ...238 Etude d’un modèle prédictif de la géométrie des pistes ...239 3.1.1
Exemple d’impression de pistes sur un objet 3D ...241 3.1.2
Impression 2D de dispositifs médicaux multimatériaux ...243 3.2
Processus de fabrication ...243 3.2.1
Obtention de propriétés barrières par dépôt d’une couche de MFC par 3.2.2
spray 246
Impression de chemins capillaires ...247 3.2.3
Résistances chauffantes imprimées ...248 3.2.4
CONCLUSION ...250 4
BIBLIOGRAPHIE ...252 5
TABLE DES FIGURES ...253 6
ABSTRACT ...254 RESUMÉ ...254
FURIA Gioia 9
GENERAL INTRODUCTION
FURIA Gioia 10
TABLE OF CONTENT
1. CONTEXT OF THE PROJECT ... 11
2. OBJECTIVES OF THE THESIS ... 13
3. MAIN OBSTACLES AND THESIS STRUCTURE ... 14
4. BIBLIOGRAPHY ... 16
FURIA Gioia 11
1. CONTEXT OF THE PROJECT
A growing demand for prototyping processes is emerging in the fields of electronics and connected objects to simplify and automate the process of integrating electronic components into 3D objects. For this reason, plastronics is developing and is really starting to appear on the market since the 2000s. [1]
This discipline, which combines plastics processing and electronics, facilitates the integration of electronics into objects in order to make them functional. To do this, certain electronic functions and links between components are no longer supported by a conventional 2D electronic board (PCB: Printed Circuit Board) but directly integrated on the 3D object.
In order to offer a versatile and easy to implement alternative for prototyping and small series, printed electronics is also widely considered. This technology consists in printing an electrically conductive ink on the surface of already formed 3D objects in order to create the electronic functions deported on the object and the links with a possible PCB board. For small series, the advantages of this technique are the following:
- No restriction of materials for the manufacturing of the object. Due to the plurality of inks (viscous, fluid, aqueous or solvent based, metallic or organic ...) and deposit systems (pressure, worm, drop ejection ...) existing, the printing of a quality circuit can be achieved on any material.
- Additive technology: on the one hand, only the necessary amount of material is used, there is no waste. On the other hand, the process is direct, the conductive tracks are created in a single step.
In the same time, the industrial robotics market is in constant evolution, there are today more than two million industrial robots in the world.
Since 1987, IFR (International Federation of Robotics) has been collecting worldwide sales data for industrial robots, so as illustrated in Figure 1, the market has more than tripled in 10 years.
FURIA Gioia 12 Figure 1: Worldwide annual supply of Industrial Robots from IFR
Many robot models have been developed to meet the requirements of new applications in terms of weight to be carried, range of motion, speed and precision.
In the industrial field, the most widespread robots are robot arms, in areas such as welding, painting or assembly.
An increasing number of high-tech sectors are starting to use industrial robots such as telecommunication, Internet of things (Iot) and additive manufacturing and many small and medium enterprises (SMEs) are wondering about the integration of robots in their structure.
However today no simple system of use is available on the market for 3D electronic printing. At the research level, developments are focused on the use of Cartesian X,Y,Z printers, sometimes with a 4th axis of rotation, supporting print heads to deposit the conductive tracks during the manufacture of the object [2], [3] or on 2,5D objects[4], [5].
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2. OBJECTIVES OF THE THESIS
The subject of the first part of the thesis, collaboration between the SME Mind and the laboratory LGP2 (Laboratoire Génie des Procédés Papetiers), is the creation of a robotic cell for the prototyping and production of small series of cellulose-based connected objects functionalized on the surface by direct circuit printing.
The printing of conductive tracks allows the integration of electronic functions directly on the surface of the object without the systematic transfer of one or more conventional 2D electronic boards and the replacement of electrical wires between components by printed conductive tracks.
All operations will be performed by 6-axis robots on which will be mounted various working tools, including a laser scanner and one or more printing heads.
The platform will be completed by a dedicated software allowing the management of the whole production process and the automatic creation of the machine code for the piloting of the manufacturing process. This software, equipped with a simplified interface and calibration protocol, will allow both the use of the prototyping line by people who are not experts in robotics and a high speed in the customization of printed circuits and product changeover.
The second part of the thesis, collaboration between Carnot Polynat and the LGP2, consist in using the developed cell for the manufacturing of multi-layers cellulose-based medical tests.
FURIA Gioia 14
3. MAIN OBSTACLES AND THESIS STRUCTURE
The goal of this project is to provide an answer to the problem of the electronic functionalization of 3D objects to make them functional by an automated process, versatile, easy to implement and compatible with prototyping and small series.
The main obstacles to achieving this goal are the development and implementation of a direct writing process on 3D objects using a six-axis multi-tool industrial robot. This lock, which represents the heart of the project, covers:
- aspects concerning the sources of inaccuracies that impact the process like the object geometry, which can be a macro-geometrical default or a positioning default between the object and the robot. Thus, a trajectory designed from a theoretical geometry and position is not necessarily valid. [6], [7]
- aspects concerning the identification/implementation of deposition techniques adapted to the process (e.g. extrusion, spray, jetting, ...) ;
- the development of a protocol for managing the movements of the robotic arm enabling the deposition of conductive tracks
The thesis will therefore be structured as illustrate in Figure 2.
After this introduction chapter, a review of the literature will be conducted (chapter 1); then two main contributions will be made:
- chapter 2 describes the integration and qualification of all the tools on the 6-axis robot as well as the creation of a demonstration cell and the dedicated control software.
- chapter 3 presents applications tested with the developed cell . Two main applications are tested, the production of small series of cellulose-based multi-layers medical tests and example of simple 3D connected objects.
FURIA Gioia 15 Figure 2 : Thesis structure
FURIA Gioia 16
4. BIBLIOGRAPHY
[1] D. Unnikrishnan, « Mid technology potential for RF passive components and antennas », Univ. GRENOBLE, p. 246, 2006.
[2] M. Ahmadloo et P. Mousavi, « A novel integrated dielectric-and-conductive ink 3D printing technique for fabrication of microwave devices », in 2013 IEEE MTT-S
International Microwave Symposium Digest (MTT), juin 2013, p. 1‑3, doi:
10.1109/MWSYM.2013.6697669.
[3] C. Shemelya et al., « Multi-functional 3D printed and embedded sensors for satellite qualification structures », in 2015 IEEE SENSORS, nov. 2015, p. 1‑4, doi: 10.1109/ICSENS.2015.7370541.
[4] B. Y. Ahn et al., « Planar and Three-Dimensional Printing of Conductive Inks »,
JoVE J. Vis. Exp., no 58, p. e3189, déc. 2011, doi: 10.3791/3189.
[5] J. Hörber, J. Glasschröder, M. Pfeffer, J. Schilp, M. Zaeh, et J. Franke, « Approaches for Additive Manufacturing of 3D Electronic Applications », Procedia CIRP, vol. 17, p. 806‑811, déc. 2014, doi: 10.1016/j.procir.2014.01.090.
[6] B. Loriot, « Automation of Acquisition and Post-processing for 3D Digitalisation », Theses, Université de Bourgogne, 2009.
[7] S. Khalfaoui, « Production automatique de modèles tridimensionnels par numérisation 3D », Dijon, 2012.
5. TABLE OF FIGURES
Figure 1: Worldwide annual supply of Industrial Robots from ... 12 Figure 2 : Thesis structure ... 15
FURIA Gioia 19
CHAPTER 1: BIBLIOGRAPHY
FURIA Gioia 20
TABLE OF CONTENT
1 INTRODUCTION ... 23
2 HIGH PRECISION FABRICATION PROCESS ... 24
2.1 Robotic arm: architecture and cinematic ... 24
2.2 Manufacturing process ... 25
2.3 Main issues causing inaccuracies ... 26
2.3.1 Inaccuracies related to the object ... 27
2.3.2 Inaccuracies linked to the process ... 28
2.3.3 Inaccuracies related to the static accuracy of the robot ... 28
2.4 Mesure in-situ ... 30
2.4.1 Position of the measuring phase in the manufacturing process ... 30
2.4.2 Integration of the measuring equipment in the working area ... 31
3 FUNCTIONAL MATERIALS PRINTING ... 33
3.1 Direct and contactless printing process ... 33
3.1.1 Aerosol ... 34 3.1.2 Inkjet ... 35 Continuous Inkjet ... 35 3.1.2.1 Drop of Demand ... 35 3.1.2.2 3.1.3 Jetting ... 36 3.1.4 Extrusion ... 36 3.1.5 Comparison ... 37
3.2 Physical and chemical properties ... 38
3.2.1 Physico-chemistry of the ink ... 38
Surface tension... 38
3.2.1.1 Colloidal stability ... 39
3.2.1.2 3.2.2 Rheological behaviour of the ink ... 40
3.2.3 Conductive property of printed tracks ... 42
Resistivity and conductivity ... 42
3.2.3.1 Quality index ... 43 3.2.3.2 3.2.4 Metallic inks... 43 Micro/nanoparticles inks ... 44 3.2.4.1 Inks based on metal salts (Metallo Organic Decomposition MOD) ... 45
3.2.4.2 Studies have been carried out by ... 45
FURIA Gioia 21 Catalytic inks ... 45 3.2.4.3
Inks causing a redox reaction ... 45 3.2.4.4
3.2.5 Carbon based ink ... 46 3.2.6 Conductive polymers ... 47 3.3 Substrates used in electronic printing ... 48 3.3.1 Substrates characteristics ... 48 Roughness ... 48 3.3.1.1 Porosity ... 49 3.3.1.2 Surface energy ... 49 3.3.1.3 Thermal stability ... 49 3.3.1.4
3.3.2 Substrates studied in literature ... 50 3.3.3 Molded cellulose ... 51 3.3.4 Micro Fibrillated cellulose (MFC) ... 52 3.4 Annealing types ... 52 3.4.1 Thermal annealing ... 53 3.4.2 Chemical annealing ... 53 3.4.3 Electrical annealing ... 54 3.4.4 Plasma annealing ... 55 3.4.5 Microwave annealing ... 56 3.4.6 Photonic annealing ... 56 Infrared annealing ... 57 3.4.6.1 Laser annealing ... 57 3.4.6.2
Intense Pulsed Light annealing (IPL) ... 58 3.4.6.3
4 CONNECTED OR FUNCTIONAL OBJECT ... 60 4.1 Molded Interconnect Devices (MID) ... 60 4.1.1 Fabrication process ... 60 Laser direct structuring (LDS) ... 60 4.1.1.1
Laser subtractive structuring (LSS) ... 61 4.1.1.2 Microstamping ... 61 4.1.1.3 Bi-injection ... 61 4.1.1.4 Inkjet ... 61 4.1.1.5
4.1.2 Research project examples ... 62 Electronic field ... 62 4.1.2.1
FURIA Gioia 22 Medical field ... 62 4.1.3.1 Automotive field ... 62 4.1.3.2 Telecommunication field ... 63 4.1.3.3
4.2 Additive manufacturing of 2D multilayer functional devices ... 63 4.2.1 Paper microfluidic ... 63 4.2.2 Papertouch ... 64 4.3 Additive manufacturing of 3D functional objects ... 65 4.3.1 Multimaterials objects additive manufacturing process ... 65 Material deposit ... 65 4.3.1.1
Photopolymerisation ... 66 4.3.1.2
Manufacturing on powder bed ... 67 4.3.1.3
4.3.2 Industrial application examples ... 67 Voxel8 ... 68 4.3.2.1 Nano Dimension ... 68 4.3.2.2 Optomec-Stratasys ... 69 4.3.2.3
4.4 Robotic for 3D printing ... 69 4.4.1 Research project examples ... 69 +Lab –Milan Polytechnic University ... 70 4.4.1.1
BatiPrint3D-Nantes ... 70 4.4.1.2
4.4.2 Industrial application examples ... 71 Stratasys ... 71 4.4.2.1 Drawn ... 72 4.4.2.2 Poietis ... 72 4.4.2.3 Bioassemblybot ... 73 4.4.2.4 5 CONCLUSION ... 74 6 BIBLIOGRAPHY ... 75 7 TABLE OF FIGURES ... 83 8 TABLE OF TABLES ... 84
FURIA Gioia 23
INTRODUCTION
1
The objective of this chapter is to analyze in depth the literature dealing with the manufacturing process of 3D molded cellulose objects including surface printed electronic circuits by robotic printing, to highlight the most influential parameters and the means to control them.
The first part describes the process of printing electronic circuits on 3D parts. It highlights the main problems encountered and presents the measurement means used to control the process as well as their integration during manufacturing.
The second part deals with the parameters involved in the printing of conductive tracks and the study of research work to understand and optimize them.
The third part focuses on the printing technologies that enable the manufacture of functionalized 3D objects, the related research topics and their main industrial applications.
FURIA Gioia 24
HIGH PRECISION FABRICATION PROCESS
2
In the literature, a lot of work exists on the analysis of robotic processes such as welding, cutting or milling [1], [2] . Robotic manufacturing processes using poly-articulated structures allow great flexibility in the design of the parts to be produced and are beginning to interest fields such as additive manufacturing and architecture. [3]
Robotic arm: architecture and cinematic 2.1
The structures of the robotic arm type have a serial architecture with 6 degrees of freedom, i.e. they are composed of 6 kinematic rotational links arranged one after the other as illustrated in Figure 3.
Figure 3: Architecture 6-axis robot [4]
The use of 6-axis structures is beginning to develop because they provide a real advantage for the development of complex parts. Academic and industrial work has enabled to propose ways of improving their performance [4] but their accuracy and repeatability are not yet equal to that of machine tools. [5]
The ability of a structure to generate motion is directly related to its architecture. Each movement is generated by the displacement of an axis composed of a control part which controls the servo-control in position and speed of the movement and an operative part composed of the motorization and guiding systems allowing the movement.
Along a trajectory, the speed variation, acceleration and jerk parameters are imposed by the manufacturer of the robotic arm in order not to overload the various elements. These parameters are implemented in the robot controller and will directly influence the speed of the trajectory.
Initially, industrial robots were programmed by manual teach-in. The programming of a trajectory was done by manually teaching each crossing point. This method was
FURIA Gioia 25 therefore not sensitive to part positioning errors relative to the robot. Then, with the arrival of robotic CAD, Offline Programming methods appeared, trajectories were created digitally and the robot had to reach theoretical positions rather than taught positions. A lot of work has been done on off-line programming methods and their optimization for industrial applications. [6], [7]
Manufacturing process 2.2
The part development processes, regardless of the process used, are relatively similar in approach. The objective is to manipulate a tool in relation to a part by means of a supporting structure.
These processes can be broken down into four steps: [8]
Design: this step consists of defining the geometry of the object and generating the CAD (Computer Aided Design) model.
Generation of trajectories: this second phase allows to define the parameters related to the process and to generate a CAM (Computer Aided Manufacturing) model.
Post-processing: during this step the generated trajectories are translated into the appropriate language for the production system.
Execution: In this last phase of execution, the instructions are sent to the system which physically performs the manufacturing operation.
By analogy we can decompose the process studied in this thesis, of robotic printing of electronic circuits on 3D objects in several steps illustrated in Figure 4.
FURIA Gioia 26
Design
Step 1: Importing the part model into a CAD software. Positioning of the model in the workspace and orientation in relation to the print head according to the surfaces to be printed.
Step 2: Drawing the electronic tracks on the CAD model.
Trajectories generation
Step 3: Definition of printing parameters and generation of trajectories.
Post-processing
Step 4: Generation of files in robot language. Import of the files into the robot software and management of the I/Os of the different sensors.
Execution
Step 5: Simulation in the robot software or in manual mode Step 6: Adjustments
Step 7: Printing the conductive tracks on the 3D object Step 8: Annealing of printed tracks
A track of study envisaged in this thesis will then be to manufacture the 3D object with an adapted print head mounted on the robot then to come as explained previously, to print the circuits on the surface. In this case the step 1 consists in drawing the 3D object in the CAD software and during the step 3, the generated trajectories will be those of the object and those of the conductive tracks.
Generally speaking, the manufacturing process involves many parameters that increase the sources of inaccuracy, which has a direct impact on the quality of the final object, especially when the process requires a high level of precision.
These parameters have been the subject of bibliographical research, presented in the following paragraphs.
Main issues causing inaccuracies 2.3
In their works, Buschhaus, Wagner et Franke, [9] decompose the total inaccuracy of a process of handling a part by a robotic arm in relation to a fixed tool into the sum of the errors related to the robot, the manipulator, the tool and the part as illustrated in Figure 5.
FURIA Gioia 27 Figure 5 :Parameters that influence process quality [9]
Thus the analysis of the manufacturing chain of the process studied in this work makes it possible to highlight the main sources of inaccuracies related to the object, the process and the static accuracy of the robot and to consider areas for improvement.
Inaccuracies related to the object 2.3.1
These may be macrogeometric defects and/or defects in the accuracy of positioning of the part in relation to the robot.
Indeed, depending on the manufacturing tolerances of the part, there may be geometric differences between the CAD model of the object and the real object. Or some objects do not have a CAD model. Thus, a path drawn from a theoretical geometry and a theoretical positioning is not necessarily valid.
In order to compensate for these defects, it is necessary to obtain a CAD model that is as real as possible in order to be able to draw accurate electronic tracks.
One possible solution is to digitize the object, i.e. to obtain a digital representation of its surface geometry in the form of a point cloud or mesh using an external sensor. A data processing system is used to obtain the 3D coordinates of the object from the raw data provided by the sensor. [10]–[12]
This implies the addition of a preliminary step more or less long depending on the level of accuracy to be achieved and the development of an additional interface to process the data retrieved by the sensor.
FURIA Gioia 28 Inaccuracies linked to the process
2.3.2
The chosen process also imposes constraints which, if not properly controlled, can lead to defects in the manufacturing process and thus to a deficient or non-functional object. In the case of printing interconnections or passive components such as RFID circuits, it is necessary to have a high degree of process control to achieve very high accuracy of printing head positioning along the trajectory.
As illustrated in Figure 6, Redinger et al. [13], [14] printed by inkjet lines with a width of 160µm with 100µm spacing for RFID system applications.
Thus, the movements of the print head, its orientation and inclination with respect to the support condition the quality of the print. So it is important to control them.
Figure 6: Inkjet printing of passive components [14]
In addition, the CAD model does not take into account the material of the object and depending on the surface condition of the object, problems may arise depending on the print head used and the required printing distance between the head and the media. Indeed, some print heads require a printing distance of a few micrometers, which is of the same order of magnitude as the surface roughness of some substrates. [15], [16]
Inaccuracies related to the static accuracy of the robot 2.3.3
In order to reduce the errors between the trajectory from the CAD and the trajectory in the real environment, a manual adjustment of the trajectory or a calibration phase before the start of the task can be considered.
Studies have been carried out on robot, workspace and tool calibration.
In the field of 3D printers, calibration procedures are proposed in particular to correct defects related to the flatness of the plate. [17]
FURIA Gioia 29 platen in 5 or 9 points. An inductive or capacitive sensor is mounted on the print head which will be placed at different points evenly distributed on the platen. The printer will make a correction relative to the flatness of the plate.
Also with the aim of improving positioning accuracy, calibration methods have been developed for 6-axis systems. The objective is to identify the actual geometrical parameters of the robot, i.e. the lengths of the arms and their orientation with respect to the axes of rotation in order to improve the accuracy of the robot end device. This may involve calibration with or without sensors.
In general, calibration involves four steps: modelling, measurement, identification and compensation. [18], [19]
Khalil et Besnard [20], [21] propose a comparison between different autonomous calibration methods without external sensors.
Calibration with multi-plane links is regularly used in research work on robotic arms [22]. This method consists in using the articular coordinates of a set of configurations in which the end of the end effector is in contact with a plane. Then, the geometrical parameters are identified using minimization algorithms. Finally, the new parameter values are integrated into the control, which compensates the precision error. As illustrated in Figure 7, calibration is carried out with a calibration block machined with small tolerances and the contact can be checked by a probe.
Figure 7: Sensor and calibration cube [22]
Other more expensive calibration methods using external measurement sensors such as theodolites, camera, laser or acoustic sensor can be used. Khalil et Dombre [23] present a comparison of these systems.
Buschhaus, Wagner et Franke, [9] propose a closed-loop calibration method, the principle of which is to send the robot to reference marks, measure the deviation and apply correction coefficients as illustrated in Figure 8.
FURIA Gioia 30 Figure 8 : Calibration method example [9]
In conclusion, to guarantee the accuracy of the robot in its work cell, it is necessary to know and control the different sources of error.
Various sensor technologies are available for measurement and control. However, in order to ensure that the control is time-efficient and allows for a high level of reactivity in correcting defects, the measurement must be integrated as far as possible into the manufacturing phase.
Measure in-situ 2.4
Position of the measuring phase in the manufacturing process 2.4.1
The measuring phase can be done post-process, after manufacturing or in situ during the manufacturing phase. [24], [25]
Post-process measurement is time-consuming because it requires the object to be moved to the measuring equipment and defects can only be detected after manufacture. However, it leaves the tool available for further production.
The in-situ measurement is carried out at the same time as the manufacturing process, without moving the object. It can be done in-process without stopping the means of production during the measuring phase or on-machine when manufacturing is stopped as illustrated in Figure 9.
FURIA Gioia 31 Figure 9: In-situ measure inspired from [25]
The in-process measurement allows to be very reactive on the correction to be made during the manufacturing process. However, it is complex to implement because the manufacturing and measurement phases must communicate and operate at the same time without risk of collision.
This method is used in the industry to monitor machining for example, because it allows to improve quality without impacting productivity. [26]
On-machine measurement, carried out when manufacturing is stopped, takes into account the measurement phase in the manufacturing phase and thus allows good reactivity in correcting defects by making corrective actions possible directly in the manufacturing environment. However, as manufacturing is stopped, productivity is reduced.
Setting up an in-situ measurement system requires taking into account certain constraints such as :
The duration of the measurement phase so that it is not limiting for the manufacturing process.
The management of the communication between the manufacturing data and the measured data so that the phases exchange and work properly.
Integration of the measuring equipment in the working area 2.4.2
Various studies have been carried out on the integration of measuring equipment during production.
Poulhaon et al. [27] propose a method to adjust the machining path in real time according to part defect measurement data. The measurement is performed In-process with a laser sensor.
FURIA Gioia 32 obtained from images of the milling tool.
Ko et al. [29] integrate a laser plane on a machine tool and present comparative results between measurements made with a Coordinate Measuring Machine (CMM) and the results obtained with the developed On-machine measuring system.
In the field of additive manufacturing, the need to control print stability has also led to various studies. [30]–[32]
Tapia et Elwany [33] and Everton et al. [34] present a review of tools, measurement and real-time control methods used in the specific case of additive metal fabrication. Sammons et al. [35] investigate the use of displacement sensors to control layer height during printing. Other work presents the use of IR sensors to monitor the temperature of materials during manufacturing. [36]
Patents have also been filed on the development of new control methods and systems. [37], [38]
The installation of measuring systems and the analysis of the resulting data allow on the one hand the improvement of the quality of the obtained parts but also the optimization and development of additive manufacturing techniques.
In conclusion, a perfect control of the stages of the manufacturing process requires the implementation of methods for measuring and controlling the manufacturing parameters. This expertise is an essential element to allow a continuous improvement of the process, to obtain high quality parts and in an industrial vision to remain competitive.
The following paragraph presents the bibliographical study of the parameters influencing the printing quality of conductive tracks.
FURIA Gioia 33
FUNCTIONAL MATERIALS PRINTING
3
Functional materials printing depend on the compatibility between four main elements: the printing technology, the ink, the substrate and the annealing method.
Direct and contactless printing process 3.1
Direct printing processes also called digital printing have been developed or have known a great evolution in the past few years because they allow to meet growing demands of flexibility, development rapidity, low cost and waste reduction. These processes are also able to reach high production volume which makes them particularly suitable for microelectronic industry.
Few definitions, very generalists have been proposed in the literature to describe direct printing process such as:
Any technique able to deposit various materials type on different substrate according to a defined pattern. [39]
Thereafter, Hon, Li and Hutching [40] propose a definition enabling the differentiation between direct printing processes and rapid prototyping processes :
All the processes able to deposit with a high precision level functional or structural material on a substrate with a digitally defined pattern.
Finally, Zhang et al. [41] present a definition combining few of the precedent definitions and define as direct printing technique any additive technique that enable the deposit of electronic components and functional pattern on various type of materials without using mask or subsequent engraving operation. The deposit of a material is followed by a sintering or drying operation in order to enable the material to reach its performances. Direct and contactless printing processes can be classified in three main printing categories by drop ejection, energy beam or material deposition; these main categories themselves subdivided in different technologies are illustrated in Figure 10 and will be described in the following sections and compared in section 3.3.1.5.
FURIA Gioia 34 Figure 10 : Different types of direct printing technology
Aerosol 3.1.1
As illustrated in Figure 11, an aerosol printing system is composed by two main elements an atomizer and a deposition head. The ink in liquid form is supplied in the atomizer where it is transformed in a dense vapor of droplets that have a size from 1 to 5µm. The vapour is then conducted to the deposition head by an inert gas flow where it is concentrated in a annular gas sheath. The jet formed is printed on the substrate.
Figure 11: Aerosol printing head functional schema [41]
This technology allows to use fluid with a wide range of viscosity from 0.7 to 2500 Pa.s and to print with a maximal speed around 10 m/min. The printing head height from the substrate can be adjusted between 1 and 5 mm and the printed lines can reached a
Direct printing process
Drop ejection
Ink-jet Aerosol Jetting
Energy beam
Laser
Filament
FURIA Gioia 35 minimal width of 10 µm with a minimal distance between lines of 20 µm. [41]
Inkjet 3.1.2
As illustrated in Figure 12, it exists two variants of inkjet process: Continuous InkJet (CIJ) and Drop of Demand (DOD).
Figure 12: CIJ printing head (A), thermal DOD printing head (B) et piezo electric printing head (C) functional schema from [42]
Continuous Inkjet
3.1.2.1
Continuous ink-jet technology is based on the ejection of a continuous flow of ink droplets. At the nozzles exist droplets are charged by an electrode and selectively deviated by the application of an electric field. The undesirable droplets are sent in a tank and recycled. The resolutions that can be reached with this technology remain limited as well as the inks that can be compatible with the application of an electric field.
Drop on Demand
3.1.2.2
The drop of demand method is based on the generation of drop just when it is needed. The ejection of a drop by the nozzle is made by an overpressure in the ink-jet head. This overpressure is created either by a thermic element in the ink container which under an impulsion vaporizes locally the ink solvent; a gas bubble is formed that create an overpressure.
Or by a piezo-electric element in a wall of the ink container chamber which is deformed under the action of an electric impulsion. The chamber volume is therefore reduced which creates the ejection of a drop.
In printed electronic, the piezo electric method is the most used because it allows to adjust the drop characteristics, size, volume and frequency by controlling the impulsion
FURIA Gioia 36 generation.
This technic also allows to reach high resolution printing with minimal lines width between 10 and 50 µm. [43]
Jetting 3.1.3
As illustrated in Figure 13, the operating principle of a jetting head is a combination between a pneumatic and a mechanic system. The ink is put under pressure and injected in the chamber. A piston controlled by a piezoelectric element open and closes the nozzle according to the signal send to the piezoelectric element.
Figure 13: Jetting printing head schema (A) anf functional cycle (B)[15] The impulsions can be divided in four steps:
- Rising phase corresponding to the time required to open completely the nozzle - Open time during which the nozzle remains open
- Falling phase corresponding to the time required to close the nozzle - Delay phase between two cycles
This technology allows the ejection of materials with a viscosity between 0.05 and 200 Pa.s and also until 2 000 Pa.s according to the suppliers. [15]
Extrusion 3.1.4
The extrusion of filament printing method is different from the other methods described before because the material flows in continue instead of being ejected in droplets form. As illustrated in Figure 14, the extrusion of ink can be done by the application of a pneumatic or mechanic pressure on a container or through a syringe that sends the ink on an endless screw. One of the main difficulties of this technique is to control the extrusion stoppage. [44]
FURIA Gioia 37 Figure 14: Functional schema of extrusion printing head from [45]
Comparison 3.1.5
As summarized in Table 1, each direct printing technology has its own characteristics in terms of minimal line width, minimal line thickness, maximum printing speed, ink viscosity and distance between printing head and substrate during printing.
The highest printing speed can be reached with inkjet and jetting printing technology. Aerosol and inkjet allow to print very thin lines.
When printing on a 3D surface maintaining a constant distance between printing head and substrate can be challenging, thus to have the possibility to print around 1 mm from the substrate enables a greater flexibility; it can be done with all these printing technology except with extrusion that requires a distance around 0,2 mm.
Finally ink with a high viscosity can be printed with aerosol or jetting printing systems.
Printing process Minimal line width (µm) Minimal Thickness (µm) Maximum printing speed (mm/s) Ink viscosity (Pa.s) Printing head to substrate distance (mm) Aerosol 10 1 10 0,7-2500 1-5 Inkjet 20 0,01 80 0,001-0,1 1 Jetting 400 20 100 0,05-200 1 Extrusion 250 10 20 0,05-2000 0,2
Table 1: Direct printing process comparison
Once the printing process has been defined, it is a question of choosing an ink whose characteristics are compatible with it and which corresponds to the desired functionality, in the case of this thesis the study deals with inks for printed electronics.
FURIA Gioia 38
Physical and chemical properties 3.2
The type of conductive ink that can be used with contactless printing process is limited by its physical and chemical properties; a presentation of the main characteristic is made in this part.
The three broad categories of ink for printed electronics are then presented: metallic inks, carbon inks and conductive polymers
Physico-chemistry of the ink 3.2.1
Whatever its nature, an ink is always composed of three elements: [46]
- The functional material: for traditional printing ink this is the colour substance in the form of pigments; for printed electronic ink this is conductive materials in the form of metallic salts, nanoparticles or polymers. It represents between 1 and 40% of ink mass percentage according to the application.
- The vehicles: it is the ink majority phase between 60% and 95% of ink mass percentage. It is composed of solvants and/or polymers. It allows the suspension of the functional material and the adjustment of viscosity according to the printing process. It serves as a binder between functional materials and support. - The additives: these elements varied in nature, they allow to adjust the ink
rheological properties and are chosen according to the application. The can represent up to 10% of the ink mass loading
The ink rheological and physico-chemical properties study is necessary to choose an ink adapted to the printing process used.
Surface tension
3.2.1.1
The surface tension is one of the main ink properties; it determines the spreading of the ink on a substrate during printing and its adhesion on the substrate.
Thus, in contact with a surface, an ink drop is in an energetically unstable state. The surface tension measures the binding energy default of a drop by surface unit. It is linked to the interactions ensuring the cohesion of fluid molecules, they can be links of different types, Van der Waals, hydrogen or ionic. The stronger the attractive interactions, the greater the surface tension. [47]
Therefore the surface tension of a liquid (γ) is defined as being the work required (W) for an increase of surface area (A).
FURIA Gioia 39 It is expressed by
γ =
𝛿𝑊𝑑𝐴
(1)
with γ (N.m-1) the surface tensionW (N.m) the provided work A (m²) the surface area
The measure of contact angle (θ) between a deposited drop and a substrate allows the prediction of the spread of the fluid on the substrate. As illustrated in Figure 15, several cases can be distinguished
- θ = 0 : the substrate is completely wet - θ<90°C : the substrate is hydrophilic
- 90°C <θ<150° : the substrate is hydrophobic - 150°C<θ : the substrate is super hydrophobic
Figure 15: Drop spreading on a substrate
Colloidal stability
3.2.1.2
Inks are composed of suspensions of particles, pigments or nanoparticles and must be stable in order to be printed.
Suspensions of particles smaller than one micrometre in size, which is the case for inks based on nanosilver particles for printed electronics, are known as colloidal suspensions.
However because of Brownian motion between particles, a colloidal suspension is never in equilibrium, as the particles tend to aggregate or sediment. It is therefore essential to stabilize these inks to allow their use.
The stabilization of nanoparticles depends on the interactions between the particles and the solvent and the attractions of the particles to each other, linked to the Van der Waals interactions.
These interactions result from the presence of different electrical polarizations between the solid particles and the liquid phase. The intensity of the attraction linked to the Van der Waals interactions varies proportionally to the difference of polarity between the phases and inversely proportional to the distance between particles.[48]
FURIA Gioia 40 Thus Van der Waals’ interaction between two colloidal spheres in solution is defined by the equation:
VVdW= − 𝐻𝑅
12𝑑 (2)
with H (J) Hamaker constant of the system R (m) particles radius
D (m) distance between particles
In order to stabilize a colloidal suspension, the particles must therefore be kept at distance. There are three types of stabilization: electrostatic, steric and electrosteric.
Electrostatic stabilization
Electrostatic stabilization is based on the electrostatic repulsion of particles of same charge. In the case of suspensions based on metallic nanoparticles, ions are adsorbed on the surface of the particles and create an electric field that causes the particles to repel each other and stabilize the suspension.
This stabilization requires a polar solvent in order to solvate the ions, the grater the ionic strength of the solvent, the more free charges in solution and the better the stabilization.
Steric stabilization
Steric stabilization is based on the steric hindrance of molecules adsorbed on the particles surface. This stabilization is used in the case of suspensions in which the solvent has a low ionic strength, the solvent must in this case swell the adsorbed molecules to allow total coverage of the particles.
Electrosteric stabilization
Electrosteric stabilization is a combination of the two previous types of stabilization. In this case the macromolecules surrounding the particles are themselves charged and the stabilization is done by steric and static repulsion. The solvent must have a polar character and allow the solvatation of the macromolecules.
Rheological behaviour of the ink 3.2.2
The rheological analysis of an ink consists in studying its deformation under shear stress. A fluid can be modelled as a stack of layers that slide relative to each other, which creates a shear stress at the interface between each layer. Viscosity (η) quantifies the flow resistance of a fluid. [49]
FURIA Gioia 41 It relates stress (τ) to shear rate (γ) by the equation: η = 𝜏
𝛾 (3)
with η (Pa.s) viscosity γ (s-1) shear rate τ (Pa) shear stress
In the case of Newtonian fluids such as water or oil, the constraint is proportional to the shear rate, viscosity is thus constant.
In the case of non-Newtonian fluids, viscosity is not constant; it depends of the shear rate. If viscosity decreases when shear rate increase, the fluid is said to be rheofluidifying and if viscosity increases when shear rate increases, the fluid is said to be rheothickening.
Finally some fluids are said to be threshold fluids, in this case the flow only takes place if a sufficiently high stress is applied.
The most commonly observed law of behaviour for threshold fluids is the Hershel-Bulkley law:
τ = τ0 + Kγn (4)
with τ (Pa) shear stress
τ0 (Pa) threshold constraint
K fluid constant γ (s-1) shear rate
n flow index
As illustrated in Figure 16when n=1 this law becomes the Bingham law and K plastic viscosity.
Figure 16: Rheological behaviour of fluids without critical stress (a) and with critical stress (b)
In the case of the suspensions, the rheological behaviour is more complex to model, it depends of parameters such as the volume fraction of the particles in suspension, their
FURIA Gioia 42 size and the nature of the interactions between particles.
For very dilute suspensions of spherical particles in a Newtonian fluid, Einstein’s law expresses viscosity as a function of the volume fraction of solid particles according to the equation:
ηrel = ηsusp
ηf =1+2,5 ϕ (5)
with ηrel (Pa.s) relative viscosity
ηsusp (Pa.s) suspension viscosity
ηf (Pa.s) viscosity of suspending fluid
ϕ volume fraction of spherical particles
This relation takes into account the effect of Brownian motion due to the agitation of the solid particles suspended in the fluid phase.
Numerous works have allowed to propose empirical laws in the case of suspensions more concentrated in solid particles in Newtonian fluids.
The Kreiger-Douherty model provides an estimation of viscosity according to the following equation: ηsusp ηf = (1- 𝜙 𝜙𝑚) -2,5 ϕm (6)
with ηsusp (Pa.s) suspension viscosity
ηf (Pa.s) viscosity of the suspending fluid
ϕ volume fraction of spherical particles
ϕm theoretical maximum volume fraction of the particles
Works have been conducted in the case of non-Newtonian suspensions but currently no theoretical model can predict their rheological behaviour.
Conductive property of printed tracks 3.2.3
Resistivity and conductivity
3.2.3.1
Electrical resistivity characterizes the ability of a material to conduct an electrical current. A low resistivity means that the material allows electrical charges to pass through.
Resistivity is defined as the inverse of conductivity: ρ = 1
FURIA Gioia 43 with ρ (Ω.m) electrical resistivity
c (Ω.m)-1 or (S.m-1 ) conductivity
The resistance of a straight piece of material is defined as: R = ρ 𝐿
𝑆 (8)
with R (Ω) resistance
ρ (Ω.m) electrical resistivity L (m) piece length
S (m2) straight section area
Quality index
3.2.3.2
In the case of RFID circuits, the performance of the inductive loop is characterized by the quality index (Q) proportional to the ratio of inductance and antenna resistance such as:
𝑄 = 2𝜋. 𝑓.𝐿
𝑅 (9)
with Q (without unit) quality index f (Hz) frequency L (H) inductance R (Ω) resistance
Metallic inks 3.2.4
Metallic inks are formulated from metal; the choice of metal is done according to the required electrical performances, its stability and its cost.
As illustrated in Figure 17, it exists four categories of metallic inks: micro nanoparticles based ink, inks based on metal salts, catalytic inks and inks causing redox reaction. [43]
FURIA Gioia 44 Figure 17: Different types of metallic inks [43]
Micro/nanoparticles inks
3.2.4.1
Micro or nanoparticles inks are composed of particles of the order of ten micrometres in size. These inks are essentially used with traditional printing process and are not compatible with ink-jet process but can be used with other direct printing technique such as filament extrusion.
The works conducted by Tricot et al. [50] have resulted in conductivities of the order of 3.106 S.m-1 for an ink based on silver microparticles printed with a worm screw pump
system on thermoplastic substrates PC and ABS.
The advantages of these inks are their low cost and the ability to be printed on rough substrates. However, their main limitation is a high deposition thickness compared to the other conductive inks.
The nanoparticles based inks are the most widespread and are the object of numerous studies because they have a high potential for ink-jet printing and allow patterns with a good level of electrical conductivity. The most widely used inks are silver nanoparticles inks since they have good conductive properties. Studies have been conducted on the use of cheaper conductive metals such as coper or aluminium, but these are not usable because they oxidise quickly at ambient air.
The works conducted by Cauchois et al. [51] have shown hat above a certain size the melting temperature of the metal decreases sharply, which is an advantage for this type of ink; indeed for these inks a lower annealing temperature could be used and therefore
FURIA Gioia 45 it allows the use of a wider range of substrate.
Recently, Albrecht et al. [52] compared conductive properties of silver nanoparticles inks printed by ink-jet and pulsed light annealing on nine paper substrates and ten polymer films. Conductivity between 10 and 40.106 S.m-1 have been obtained.
The main limitation of nanoparticles based inks is their formulation complexity due to the addition of stabilizer to prevent nanoparticle aggregation and stabilize the colloidal suspension.
Inks based on metal salts (Metallo Organic Decomposition MOD)
3.2.4.2
MOD inks are composed of metal salts in high concentration dissolved in organic or aqueous solvents. After printing, an annealing step causes the salts to decompose into conductive metal. The most commonly used salt is a silver complex, which gives conductivities close to those of mass silver.
Studies have been carried out by Wu et al. [53] on the formulation of metallic salts
allowing annealing at a lower temperature of about 100°C.
Valeton et al. [54] studied the room temperature UV annealing of a silver salt ink and obtained conductivities of 6,3.106 S.m-1.
Catalytic inks
3.2.4.3
The working principle of a catalytic ink is to use a reducing agent to convert metal ions into metal. Palladium is the most commonly used catalyst because it allows a very fast reaction. Works have been done by Byeon et Roberts [55] on the use of cheaper catalysts, so by using silver as catalyst they obtained a conductivity of 12,7.106 S.m-1 .
Inks causing a redox reaction
3.2.4.4
Jet-ink printing systems with two ink droplet generation channels have been developed to separately eject a metal ions solution and a reducing agent solution. When two drops contact the substrate, a metal film is formed as a result of a redox reaction. The works of Kao et al. [56] has shown that the reaction takes place within seconds and results in films with conductivities of 8. 106 S.m-1.
FURIA Gioia 46 Carbon based ink
3.2.5
Carbon based ink were the first conductive ink to be developed. Their conductivity is relatively low, so research is focusing on inks based on different forms of carbon particles illustrated in Figure 18, graphite, graphene, carbon nanotubes and fullerenes.
Figure 18: Carbon molecular structure [57]
Carbon nanotubes (CNT) are cylindrical forms of carbon, they allow to reach theoretical conductivity value close to those of metals, but after implementation in the fabrication of a conductive ink, the maximum measured values are around 10.102 S.m-1.
CNTs are used in the manufacture of transparent electrodes, transistors for RFID chips and sensors. [58]
The graphene discovered more recently is a hexagonal monoplanar structure and has a high potential due to its intrinsic conductivity close to the best metals. The average measured conductivity is around 30.102 S.m-1 but some studies have obtained
conductivities of 25.103 S.m-1. The applications are similar to those of CNT. [59], [60]
Graphite is a material of lower cost composed of stack of graphene sheets. In battery anodes, it is used as active material and is added of carbon black nanoparticles to increase conductivity. [61]
FURIA Gioia 47 Conductive polymers
3.2.6
Currently some conductive polymers offer a good compromise between speed of chemical synthesis, cost and conductive properties.
PEDOT:PSS is the most widely used as it has a high conductivity of up to 14.104 S.m-1,
polyaniline (PAni) and polypyrrole (PPy) are also used. Their chemical formulations are presented in Figure 19.
Conductive polymers are used for applications such as batteries, OLED (Organic Light-Emitting Diode) displays and organic solar cells.
Figure 19: Chemical formulations of conductive polymers from [62]
In summary, a comparison of the electrical conductivities of the different types of metallic ink presented in Table 2 shows that inks based on silver micro or nanoparticles are the most efficient.
FURIA Gioia 48
Type of inks Conductivity Applications
Micro /Nanoparticles based inks 31 106 S.m-1 (Kamyshny et al. 2005) Sensors RFID NFC
Inks based on metal salts 6 106 S.m-1
(Valeton et al. 2009) Catalytic inks 12 106 S.m-1 (Byeon et Roberts 2012) Redox inks 8 106 S.m-1 (Kao et al. 2011) CNT 10 102 S.m-1 (Loiseau et al. 2006) Electrodes
Transistors for RFID Chips
Sensors
Graphene 25 103 S.m-1
(Castro Neto et al.)
PEDOT:PSS 14 104 S.m-1
(Kim et al. 2011)
OLED,
Organic solar cells
PAni 3,67 102 S.m-1
(Xu et al. 2014)
PPy 22 102 S.m-1
(Feng et al. 2014)
Table 2: Characteristics of various types of inks
Substrates used in electronic printing 3.3
The substrate is the final key element in the printing process and its properties have a major influence on the print quality.
The most commonly used substrates in the field of printed electronics today are plastic substrates.
Substrates characteristics 3.3.1
Several characteristics are to be taken into account in the choice of substrate.
Roughness
3.3.1.1
The surface of a part is never perfectly smooth; it depends on the manufacturing process, the tools used to manufacture the parts and the material.
FURIA Gioia 49 determined by measuring different parameters illustrated in Figure 20.
The arithmetic mean deviation Ra is the most widely used indicator of roughness, it is defined as the arithmetic mean of the deviations from the mean line and is expressed in µm.
The roughness of the substrate influences the definition of the printing and must be adapted to the printing height and therefore influences the choice of printing system.
Figure 20: Roughness definition criteria
Porosity
3.3.1.2
The porosity is a numerical value expressed between 0 and 1 equal to the ratio between the void volume and the total volume of the substrate.
It has a direct impact on the penetration of the ink into the substrate.
Surface energy
3.3.1.3
Surface energy is the force that exists at the interface between two different substrates. In order to achieve good ink spread and adhesion between the ink and the substrate, the surface tension of the ink must be lower than the surface energy of the substrate.
Thermal stability
3.3.1.4
The thermal stability of the substrate, i.e. its resistance to temperature, is an important criterion for optimising the annealing step required after printing conductive inks to achieve optimum electrical properties.
Finally, the cost and the renewable or recyclable aspect of a support are also to be taken into account in the choice of substrate material and quality according to the desired application.
FURIA Gioia 50 Substrates studied in literature
3.3.2
Studies are currently focusing on increasingly complex substrates and flexible media for printed electronics; in particular on plastic substrates such as Polyethylene Terephthalate (PET), Polyethylene Naphthalate (PEN) and Polyamide (PI). Plastic substrates have good surface properties with roughnesses of a few nanometres; some plastics also achieve optical properties equivalent to those of glass.[63]
Paper is an interesting medium with many advantages such as low cost and, depending on the type of paper, resistance to temperatures of around 200°C. However, it is still little used because of its surface properties, it is indeed a rough support with a high porosity. Recent studies have led to the development of papers suitable for electronic printing.[64]–[67]
In their works Ihalainen et al. [68] study the physical properties of different papers and analyse their effects on the print quality and electrical performance of printed patterns. They obtained conductivities equivalent to the results obtained on plastic films and printed a transistor prototype.
There are also research topics on substrates such as glass, textiles or ceramics. The characteristics of the various substrates studied in literature are summarized in Table 3.
Substrate Roughness (nm) Sintering Temp (°C) Surface energy (mJ.m-2) Transparent References
Glass 1 600 50-70 Yes Chou et al 2009
Ceramic 200 1500 30-60 No Faddoul 2012 PET 3 70 44 Yes MacDonald et al. 2014 PEN 5 120 40 Yes PI 1,5 270 47 Yes
PW Paper 10 000 140 No Harting et al. 2009
PowerCoat 30 200 36 No Thenot et al. 2014
Photo Paper 10 150 40 No Kawahara et al. 2013 Filter Paper 20 000 140 No Anderson et al. 2012
Table 3: Substrate characteristics
FURIA Gioia 51 both print on 3D objects based on recyclable materials, therefore the support used for this thesis is molded cellulose; it is presented in the following paragraph.
Molded cellulose 3.3.3
Molded cellulose is made from recycled fibres, mainly from cardboard boxes, newspapers and water. Depending on the application, additives can be added in order to obtain particular characteristics in terms of strength, impermeability or colouring. Molded cellulose manufacturing process is divided into four steps:
Pulping
Cellulose pulp is prepared by suspending recycled fibres previously washed in water, this is the pulping operation.
Molding
The cellulose pulp is then moulded, resulting in a fibrous mat with the shape of the mould.
Two molding technologies exist in industry:
a flat process, in which a perforated mould is dipped into the liquid paste, under the effect of suction, the fibres are deposited on the mould while the water drains out through the holes. A fibrous mat of the shape of the mould is obtained which is transferred to a conveyor.
a rotary process, in this case the moulds are placed on a multi-sided drum, which is rotated in the dough. A suction system draws the dough into the moulds, and when they come out of the preparation a counter-mould comes to press the objects.
Pressing
The formed object is then pressed; this step aims to stabilize the object dimensionally in order to ensure accurate dimensions and a good surface finish.
Drying
The object is then dried. Drying can be done either directly in the mould, it is done by increasing the temperature and pressure of the mould. Either by one or more passages in a tunnel, the barely pressed objects are placed on a conveyor belt and taken to an oven.
